One is loathe to admit to one's mistakes. For surely, oh surely, it is our evolutionary destiny to prove our parents wrong? That generation of swingers who lustily enacted gender stereotypes, reproduced in their 20s, and submitted their offspring to a sleeping-and-feeding routine seemingly modelled on a prison timetable, now seems amusingly antediluvian.

Meanwhile, we bask in the glory of our own little Age of Enlightenment, in which frazzled, NCT-fed quadragenarians give birth with the aid of emergency medical intervention, feed on demand all night, return - still nauseous with sleep-deprivation - to reduced career prospects at five months, suffer a couple of miscarriages while they're at it, and squeeze out another one if they're lucky. Or inject possibly carcinogenic hormones into their buttocks in the hope of beating statistics. Or realise that they forgot to have babies somewhere along the way, and it's a bit too late.

Possibly, just possibly, we have got it wrong. We are the generation of have-it-alls, the daughters of feminism who combine careers and economic independence with - hopefully, eventually - babies. We're aware that it's a juggling act that leaves us on the verge of collapse; still, we're grateful for it. The advantages have scarcely been questioned. Yet the news now coming in for last-minute mothers is distinctly worrying. Reports are stressing, controversially, that by delaying motherhood, many women are simply leaving it too late. Such claims may smack of a backlash, and the figures can be misleading, yet they contain biological facts that are hard to ignore.

The average woman has her first baby at nearly 30, and the pregnancy rate among women aged 40 and over has risen by more than 40% in the past decade, while in the US, the number of first-time mothers in their 30s and 40s has quadrupled since the 70s. It is generally agreed, however, that at 30, a woman's chance of conceiving begins to decline - the journal Human Reproduction, it was reported this week, has found that even before they're 30 women are less fertile than they were at 20. By 35, the chance of conceiving falls away sharply at a rate of 5% to 10% a year. Miscarriage rates for the over-40s stand at 40%.

But what is more astonishing than the figures themselves is the fact that the assumptions of most women in their 30s - or even 40s- remain unchanged. This calm, indeed blinkered sense of entitlement is exacerbated by the availability of IVF, a process that has changed over two decades from tabloid freak show territory to an everyday part of our lives. We have reached a juncture in which culture and technology have meshed to create a particular attitude, an attitude so entrenched that there is now an expectation that we can have babies virtually whenever we wish. Even childless fiftysomethings, deriving their beliefs from both freak Italian birth stories and the example of late-sprogging Hollywood doyennes, speak calmly of ICSI, where sperm can be extracted from the testes and used to fertilise an egg, frozen eggs, donor eggs, and other infant-producing miracles. It is going to take some time for the facts to trickle into the fertility fantasy world in which we all live. Is this the generation that has misjudged it?

"I have to be honest and say that I really do wish that I'd done this 10 years ago," says Lamorna Reid, 38, who has been trying to become pregnant for the past eight months. "It's not just about conceiving (though I'm agonisingly aware that I may have played the clock just a bit too long); it's also that I feel markedly more tired and older than I did a decade ago. But the worst is not knowing whether I'll be able to have a child at all." Anna Drake, who had her first baby at 37, says: "I wish, wish, wish I'd thought about having kids earlier. It was just that I assumed that life as I knew it would end (it did) - but I had no idea how lovely parenthood would be! There were also a load of very valid practical reasons. But I'm already in the grip of a fresh onslaught of broodiness, facing 40, and I'm beginning to question the merits of delayed motherhood. I grew up totally knowing I wanted babies, yet with an equally fixed belief that I'd have them later."

Neatly reversing an earlier generation's attitudes, the precedence of career and financial independence now goes unquestioned for those who grew up, endowed with the legacies of feminism, in the careerist climate of the Thatcher years. And why shouldn't it? Women's meagre portion of financial power has been so hard-won; so many bloody battles lie behind the position we're now in. Yet state childcare, proper paternity leave, and sexual equality are clearly far from imminent, and while the truth about fertility is sickening and unjust, the bigger shock of age- related infertility is a greater tragedy.

As Pamela Madsen of the American Infertility Association chillingly commented during a fertility awareness campaign last year: "I don't want to speak to any more sobbing 43-year-olds who say, 'Nobody ever told me'. " A reluctant compromise may be the only way.

"My concern is that women are caught between a cleft stick," says Robert Forman, director of the infertility clinic CRM. "On the one hand, quite rightly, they want to advance their careers; on the other, quite often, when they've reached that stage, their fertility is significantly reduced. And, often they aren't aware of the fertility problems associated with increasing age."

It's common to encounter women aged 40 and above whose answer to the eternal, "Do you want a baby?" question is a calm, "Well, I might ... " What? Where is the sense of urgency? In ovum years, age 40 is a lifetime away from 35. The mental safety-net of IVF has so infiltrated the collective consciousness that it is now perceived as the automatic next step. Couples try for a few months, then make an appointment with the nearest clinic. But again, the figures fail to support our cultural assumptions. The majority of couples who try IVF fail. And for women aged 39-40, the chances of an IVF cycle resulting in a live birth are only 9%. Hope, supported by a lucrative fertility industry, distorts the figures, and leaves women victims of the cruellest trick of all. The links between hormone treatments and cancer are still debated, yet very much under wraps.

"Ultimately," says Lamorna Reid, "I suspect that there's something very inevitable about women delaying childbearing - at least in the society we live in now. Although I catch myself wishing I had another 10 fertile years, I know that I would just take it to the brink of whatever timescale I had. If we have autonomy and choice in our professional, social and reproductive lives, this is what will happen."

The reasons for the delay in first-time motherhood are manifold - later marriage, middle-youth attitudes, finances, lifestyle, the social pressure to put work first - and not exclusively the career preoccupation that the Daily Mail would have us believe. The culturally licensed refusal by the post-baby-boomer generation to grow up means that vast swathes of society believe that parenthood will wreck their lives. But it's not until it actually happens that the universal truth hits home: the wonders outweigh the horrors. What do we actually gain by waiting so long? Career options for women are statistically diminished at whatever age we first give birth.

Men's biological clock is also, belatedly, entering the frame: older fathers have been found responsible for genetic mutations that can mean a higher chance of mental and physical disorders in the child. Yet in the past two decades, the number of children born to fathers over 40 has risen by nearly a third. Once again, cultural trends are pitted against biology.

What Sylvia Ann Hewlett found in Baby Hunger, published in the UK this week and already the subject of much dinner-party debate, is that the more successful a woman is, the less likely she will have a partner or baby, while the converse is true for men. Hewlett claims that childlessness is becoming the norm (often reluctantly) for a generation of successful women. There is a feeling that the pendulum is about to swing back again, with women in their early 20s aware of the high price that exhausted quadra-genarians are paying. Perhaps the next generation will mock the IVF-happy, sling-toting geriatrics, just as we now pity the aproned victims of an earlier era.

It is our right to be mothers if we want to be, yet there is a danger that an entire generation is being lulled into believing it can reset the biological clock. As Robert Forman says: "By all means, balance your reproductive desires and your career needs, but put your reproductive needs quite high up in the equation: they're just as important." Otherwise, we may just find ourselves throwing out the baby.

· The Baby Hunger debate, with Sylvia Ann Hewlett, is at the London School of Economics on May 8, at 7pm. To book, call 0207 224 2295.